The present invention relates to nuclear reactors, especially of the pressurized water type in which the reactor core coolant fluid or so-called primary fluid circulates under the action of a circulating pump within at least one primary loop between a steam generator and a pressure vessel which contains the reactor core. The steam generator comprises a bundle of tubes through which the primary fluid is circulated and exchanges heat with a secondary fluid usually consisting of water which enters the generator in the liquid state and is discharged therefrom in the form of steam. After expansion within turbines and recovery in a condenser, the steam is then returned into the generator.
In order to carry out the circulation of primary fluid under pressure, each primary loop usually comprises a pump driven by an electric motor at a constant speed. However, this conventional solution is subject to a number of disadvantages, in the first place during normal operation by reason of the lack of flexibility of the motor-pump assembly and in the second place when certain fault conditions arise. Despite the presence of a substantial safety margin, it proves difficult under such conditions to maintain sufficient cooling of the reactor until the moment when the emergency systems provided can come into action.
The present invention relates to an improvement in pressurized water reactors of the type mentioned above. The essential aim of the improvement is to dispense with the use of an electric motor for driving the primary fluid circulating pump and consequently to overcome the disadvantages mentioned above. The arrangements which are contemplated permit direct utilization of the power available in the secondary fluid circuit in order to drive said pump and also to ensure operation of essential emergency devices in the event of failure or operating incidents in the primary circuit.
To this end, the improvements under consideration are essentially distinguished by the fact that the primary fluid circulating pump is driven by a turbine supplied with steam taken directly from the steam generator or from a main duct through which the steam is discharged from said generator. The turbine outlet is connected to the feedwater tank of the water unit of the main turbine by means of a pipe provided with isolating valves and a bypass pipe. In the event of closure of said isolating valves, said bypass pipe permits a flow of steam which is capable of driving an auxiliary turbine. A pump driven by the turbine serves to inject emergnecy water at high pressure into the primary circuit from a storage tank and discharge then takes place directly within the containment vessel.
In accordance with yet another distinctive feature and in the event of tripping of the main turbine, the feedwater tank is adapted to receive part of the main steam flow discharged from the generator. The secondary water under pressure of the tank can in this case be employed in two different ways. Thus in one case it can be returned directly into the steam generator via the emergency feed circuit of this latter and can continue to contribute to the production of steam which is necessary for operation of the primary turbo-pump; this procedure remains practicable even during the first stage of a primary circuit accident which has resulted in isolation of the exit steam lines from the containment vessel. Alternatively, said secondary water can be utilized as driving fluid for a secondary water injector and obtained from a storage tank within at least one steam generator and for at least one primary water injector of the emergency low-pressure injection circuit; this procedure can be adopted in the second stage of an accidental sequence of the primary circuit after the emergency high-pressure injection pumps have ceased to operate.
For the purpose of starting-up the primary pumps, the primary turbo-pumps can be supplied at a very low rate of flow by the steam derived from an auxiliary generating unit in accordance with the arrangements made for general reheating of the installation, the supply being effected by means of an auxiliary pipe having a very small diameter.
As an advantageous feature, the turbine for driving the primary fluid circulating pump is of the back-pressure type and comprises a regulating valve having at least two positions: one position permits a minimum flow of steam whilst the other position permits the normal flow with automatic return to the first position in the event of overstepping of a predetermined threshold. This valve can also serve to regulate the intake flow to the turbine between these two extremes and thus to vary the speed of the pump in a continuous manner.